Materials other than liquid crystals may be contained within droplets formed according to the method of the present method. Such material may be any organic liquid and preferably has a low water solubility, as discussed herein below. The liquid material may also be a solution of a material which is normally a solid at room temperature. One or more materials may be used in combination with, or in place of, a liquid crystalline material. As used herein, the term "organic liquid" includes reagents, adjuvants, and other chemically or biologically active species. Examples include inks, tots, dyes, flavors and fragrances. Other examples include biocides such as pesticides, herbicides, mildicides, insecticides and fungicides, marine anti-fouling agents, pharmaceutically acceptable agents, and the like. The organic liquids used in this manner according to the present invention may be pure liquids, mixtures or solutions of solid or liquid species in organic solvents. The organic liquid may be removed by evaporation, for example during film formation, leaving a void, or air or another gaseous material, within the particle.
Alternatively, material contained within the droplets may be inorganic or partially inorganic in nature, or may be comprised of precursors of inorganic species. For example, appropriately functionalized organic species could be chemically, or otherwise, converted to inorganic salts or complexes while in the droplet. Such appropriately functionalized organic species could themselves be part of a mixture or solution with one or more additional liquid or solid species. Complexes of organic ligands with metals may also be incorporated into the droplets. As discussed herein, the method of the present invention, is particularly useful in forming uniformly sized polymer particles containing liquid crystal material.
Applications for liquid crystals include: computer display screens; wristwatches; architectural windows; privacy windows; automotive windows; automobile sunroofs; switching devices such as for optics systems, projection display devices; reflective display devices; hand-held paging devices; cellular phones; laptop computers; television screens including car-mounted television screens; automotive displays including radio, dashboard, and on-board navigation systems; helmet displays such as "heads-up" displays; cockpit displays; imaging devices; virtual reality devices; simulation devices; electronic gating devices; diffraction gratings; and calculators. In these applications and others, the liquid crystal devices may be monochromatic or polychromatic.
Liquid crystal molecules are generally rod-like or disk-like in shape, and have physical properties which are intermediate between those of a crystalline solid and those of an amorphous liquid. The molecules are generally substituted biphenyl or triphenyl, wherein one of the phenyl groups may be separated from the other or others by a spacer group. Examples of liquid crystal materials include: compounds having the formula EQU A--(Ph).sub.m --X--Ph--B
wherein m is an integer from 1 to 5; A and B are independently selected from: C.sub.1 -C.sub.8 alkyl groups; halogens; cyano groups; haloalkyls; vinylene, acetylene, azo, and azoxy moieties; alkoxy groups having the formula C.sub.n H.sub.2n+1 O, wherein n is an integer from 1 to 8, and ester groups having the formula C.sub.n H.sub.2n+1 COO, wherein n is an integer from 1 to 8. X is a spacer group selected from: C.sub.1 -C.sub.8 alkylene esters; C.sub.n H.sub.2n alkyl, wherein n is an integer from 1 to 8; vinylene, acetylene, azo and azoxy groups; and azomethine linkages.
Other examples include biphenyl compounds having the formula A--Ph--Ph--B, wherein A and B are independently selected from: cyano group; halogens; alkoxy groups having the formula C.sub.n H.sub.2n+1 O, wherein n is an integer from 1 to 8, and ester groups having the formula C.sub.n H.sub.2n+1 COO, wherein n is an integer from 1 to 8.
Triphenyl liquid crystal materials may have, for example, halogen, cyano, haloalkyl and alkyl substituents. Another example of a triphenyl liquid crystal material is 4-cyano-4'-(4-[n-pentyl]phenyl)biphenyl. Biphenyl and triphenyl liquid crystal materials are well known in the art and are commercially available.
Common liquid crystal materials are rod-like molecules which can be aligned by an electric or magnetic field, or by a surface. Conventional liquid crystal display (LCD) devices rely on the ability of liquid crystal molecules to align with electric fields and surfaces. Polymer dispersed liquid crystal (PDLC) devices, discussed below, rely upon the ability of liquid crystal (LC) molecules to align with electric fields and with surfaces, and also upon the fact that the extent to which the liquid crystal molecules refract, or bend, light is dependent upon the orientation of individual molecules with respect to incident light. An indicator of the capacity of the molecules to refract light is the index of refraction, or refractive index, discussed below.
Liquid crystal devices, or LCDs can contain a range of liquid crystal types that include but are not limited to nematic, twisted nematic, super twisted nematic, cholesteric, ferroelectric liquid crystals. Such types are well known in the art.
Another type display utilizing liquid crystals relies on liquid crystal domains dispersed within a polymer matrix. A liquid crystal domain is a region occupied exclusively or predominantly by liquid crystal molecules. This type of liquid crystal display is known as a polymer dispersed liquid crystal display (PDLC). PDLCs are often used in the form of thin films, meaning films having a thickness of up to about 200 microns, typically a thickness of between 2 microns and 50 microns. The ability of the PDLC device to transmit light, ("on state") or scatter light (in the opaque "off state") is dependent upon the relative ability of the LC domain and the polymeric phase to refract light, as indicated by the so called refractive index. The refractive index of a material is the ratio of the velocity of light in a vacuum to the velocity of light in the material. The angle of refraction varies with the wave-length of the light used. The refractive index is typically represented by h, with a superscript usually added to indicate the temperature at which a measurement is made and a subscript is used to indicate the wavelength of the light source. (The Chemist's Ready Reference Handbook, G. J. Shugar and J. A. Dean, McGraw-Hill, Inc., New York, 1989). For a typical organic material, the refractive index may range from about 1.4 to about 2, and is calculated by the formula EQU h=c/v
where h is the refractive index, c is the speed of light in a vacuum and v is the speed of light in a given material.
The ability of an electric field to influence the extent of refractive index matching is due to the fact that liquid crystals exhibit differing indices of refraction, dependent upon their orientation with respect to incident light. When light passes through a liquid crystal medium along the long axis of the molecules, the refractive index measured is called its ordinary refractive index. When light passes through a liquid crystal molecule perpendicular to its long axis, the refractive index measured is defined as its extraordinary refractive index. In a PDLC device, the orientation of the liquid crystal molecules with respect to incident light is affected by the presence or absence of an electric, or alternatively magnetic, field such that the liquid crystals will express an ordinary refractive index or an extraordinary refractive index as a function of the applied field being either on or off, respectively. In the instance where the ordinary refractive index of the liquid crystal droplets is matched closely with the refractive index of the surrounding polymer matrix, light incident on a film comprising liquid crystal droplets and polymer is not refracted and the film is substantially transparent. In the instance where the extraordinary refractive index of the liquid crystal droplets does not match the refractive index of the surrounding polymer matrix, and the LC droplets are provided with the correct size and geometry, light incident on a film comprising liquid crystal droplets and polymer is refracted and the film is substantially opaque.
The size of the LC droplets, or alternatively referred to herein as LC domains, also has a profound effect upon the electro-optical characteristics of PDLC films. When LC and surrounding polymeric matrix have the same refractive index, the film will be transparent regardless of the size of the domains. When the refractive indecies of the LC droplets and the polymer matrix are not matched, however, domain size, domain size distribution, and number of domains determine the extent to which light is scattered. Ideally, the polymeric matrix of a PDLC is chosen such that its index of refraction matches the ordinary index of refraction of the LC domain.
As described hereinabove, when both LC and polymer have the same refractive index, transparency is achieved. The closeness of the matching of the indices, or the "index matching", may be selected based on the desired degree of contrast and transparency in the device. Mismatched refractive indices cause light to scatter with the result that opacity is achieved. For transparency, the ordinary index of refraction of the liquid crystal and the index of refraction of the polymer will preferably differ by no more than 0.03, more preferably no more than 0.01, and most preferably by no more than 0.001. The acceptable difference will depend on LC domain size. LC domain sizes on the order of the wavelength of the light being refracted will give maximum scattering. Domain sizes less than about 1/10th the wavelength of the light being refracted will not scatter light significantly even if the refractive indices of LC and polymer are mismatched.
A disadvantage of conventional PDLCs is an undesirably high switching voltage. Switching voltage is a voltage that is required to orient the LC molecules normal to the substrate conducting surfaces, thereby creating a transparent state. This voltage, V.sub.ON, is typical from 70 to 100 percent (V.sub.70 -V.sub.100) of the transition in transparency from the film's most opaque state to its most transparent state, preferably from 75 to 95 percent and most preferably from 80 to 90 percent of the transition in transparency from the film's most opaque state to its most transparent state. Similarly, there is a voltage, V.sub.OFF, for which the film will be in a relatively opaque state. Typically voltage V.sub.OFF is from 0 to 30 percent (V.sub.0 -V.sub.30) of the transition in transparency from the film's most transparent state to its most opaque state, preferably from 5 to 25 percent and most preferably from 10 to 20 percent of the transition in transparency from the film's most transparent state to its most opaque state.
Commercially available PDLC devices require switching voltages of about 40 volts. These voltage requirements exclude PDLC devices from many applications. Preferably, switching voltages of about 8 volts are desirable in order to access a broader range of applications. It is desirable to have as low a switching voltage as possible, in order to reduce energy requirements and to increase battery life.
The surface of the polymeric wall surrounding an LC domain in a PDLC film exerts a force upon the liquid crystal molecules of that domain such that liquid crystal molecules in contact with the polymer surface, or substantially close to the polymer surface, will behave differently in the presence and the absence of an applied field than those liquid crystal molecules not in contact with or substantially close to the polymer surface. The magnitude of that force, called the anchoring force, depends upon, for example, liquid crystal and polymer composition, but it also depends upon proximity of LC molecules to the polymer wall. The anchoring force experienced by an LC molecule decreases as its distance from the wall increases.
The polymer-wall surface area encountered by liquid crystal molecules in PDLC films depends on size of the liquid crystal domain (the term "wall" is used to describe that area of the polymer in contact with the liquid crystal domains). For example, two PDLC films, each of which contains the same total volume of spherical LC domains with each film containing spherical LC domains having a single diameter, and the LC domain diameter in one film is twice that in the other. In the film containing the smaller domains, there will be eight times as many domains as in the film containing large domains. The total wall surface area of the set of small domains will be twice the total wall surface area for the large domains due to the dependence of surface area upon the square of the radius. All other things being equal, a PDLC film containing smaller domains will have a larger fraction of its LC molecules in close contact with the polymer wall, and will require more applied voltage to align the LCs parallel with an applied field than is required for LC molecules in larger domains.
Conventional PDLC preparative methods resulted in broad distributions for the shapes and sizes of LC domains. This is true for all of the phase separation techniques. Commonly used phase separation techniques are thermally, solvent, and polymerization induced phase separation, known by the acronyms TIPS, SIPS, and PIPS, respectively and are well known in the art. Such phase separation usually occurs via one of two fundamental processes, nucleation and growth or spinodal decomposition. In the former, domains nucleate at different times and grow at different rates to give a broad distribution of domain sizes. In the latter, nearly instantaneous phase separation produces lacy bicontinuous structures which, by their very nature, exhibit a variety of shapes and sizes. Techniques involving aqueous emulsions of liquid crystal molecules, e.g., known in the art as the so called NCAP process for forming Nematic Curvilinear Aligned Phases, also produce broadly distributed LC domain sizes characteristic of mechanical emulsification. (P. S. Drzaic, "Liquid Crystal Dispersions" World Scientific, River Edge, N.J., 1995.)
As a consequence of having broadly distributed LC domain sizes, as one applies a field of increasing voltage to a PDLC film, the largest domains and the most spherical domains, i.e., those having the lowest ratio of surface area to LC volume, align normal to the applied field first. Progressively smaller and less regularly shaped domains then begin to align normal to the applied field (also known as "switching") at higher and higher voltages until, finally, the peak voltage (the lowest voltage giving maximum transparency) is reached. In this way, the presence of many domain sizes in a single PDLC film leads to a broad transition in the graph of light transmission versus voltage. Such broad transitions have excluded PDLC films from use in applications, such as pagers and cellular phones, where multiplexing, discussed below, is highly desired. Additionally, forward and backward scatter of light, off-angle haze in the electrically ON state, and wave length dependent optical properties are extremely sensitive to LC domain size.
Hand-held devices such as pagers and cellular phones have relatively simple displays, generally comprised of a single row of alphanumeric characters, i.e., letters such as, D, X and Z as well as numbers such as 1, 7 and 9, for example. Each character is made up of pixels. Various techniques for applying a voltage to one or more pixels in a plurality of pixels comprising a display application include, but are not limited to: direct drive, passive matrix addressing, and active matrix addressing. These techniques are known in the art. Conventional PDLC technology is not applicable to the use of passive matrix addressing. The following relationship is known as the iron law of multiplexing. (P. M. Alt and P. Pleshko, IEEE Trans. Elec. Dev. ED-21, 146 (1974)) EQU N.sub.max =[(s.sup.2 +1)/(s.sup.2 -1)].sup.2 where s=V.sub.ON /V.sub.OFF
Multiplexing allows to greatly reduce the number of display interconnections by addressing matrix row and column electrodes rather than individual pixel electrodes. N.sub.max defines the maximum possible number of row electrodes that can be addressed.
Convential PDLC devices transition from V.sub.OFF to V.sub.ON over a broad range of voltages with N.sub.max of 3 or less. The ability to have N.sub.max of 4 is highly desirable thereby providing PDLC devices suitable for passive matrix addressing and therefore a highly desired display technology for such applications as pagers and cellular phones utilizing English language characters. PDLC devices with N.sub.max of about 8 or greater would enable use of PDLC devices for displaying more complex characters such as Kanji characters.